Vehicle operators use interior and exterior rearview mirrors to view scenes behind the vehicle without having to face in a rearward direction and to view areas around the vehicle that would otherwise be blocked by vehicle structures. As such, rearview mirrors are an important source of information to the vehicle operator. Bright lights appearing in a scene behind the vehicle, such as from another vehicle approaching from the rear, may create glare in a rearview mirror that can temporarily visually impair or dazzle the operator. This problem is generally worsened during conditions of low ambient light such as occur at night, when the eyes of the vehicle operator have adjusted to the darkness.
Various solutions have evolved to deal with the problem of glare in automotive rearview mirrors. Initially, a prismatic mirror was provided that could be manually changed between a setting of high reflectivity and a setting of low reflectivity. When the operator experienced glare, the operator could manually change the rearview mirror setting to low reflectivity to reduce the glare. The operator could then manually switch the rearview mirror back to high reflectivity after the glare had subsided. Difficulties with manually controlled mirrors include the glare experienced before the mirror could be switched as well as operator distraction caused by finding and operating the mirror control.
Automatically dimming rearview mirrors were designed to eliminate the need for the operator to manually switch the mirror. The earliest designs used a single glare sensor facing rearward to detect the level of light striking the mirror. This design proved to be inadequate since the threshold perceived by the operator for dimming the mirror, known as the glare threshold, varied as a function of the ambient light level. An improvement included a second light sensor for detecting the ambient light level. The glare threshold in these systems is based on the amount of ambient light detected. Among the dual sensor designs proposed include those described in U.S. Pat. No. 3,601,614 to Platzer; U.S. Pat. No. 3,746,430 to Brean et al.; U.S. Pat. No. 4,580,875 to Bechtel et al.; U.S. Pat. No. 4,793,690 to Gahan et al.; U.S. Pat. No. 4,886,960 to Molyneux et al.; U.S. Pat. No. 4,917,477 to Bechtel et al.; U.S. Pat. No. 5,204,778 to Bechtel; U.S. Pat. No. 5,451,822 to Bechtel et al.; and U.S. Pat. No. 5,715,093 to Schierbeek et al., each of which is hereby incorporated by reference.
Another related approach is described in U.S. Pat. No. 5,760,962 to Schofield et al., which is hereby incorporated by reference, in which an imaging array is used to gather light from behind and beside the vehicle. Ambient light is detected by examining pixels generally looking sideways. The cost of the imaging array, the required lens, and complicated signal processing logic make the use of an imaging array prohibitively expensive for many automotive applications. Also, light collected from a side view less accurately represents the ambient light experienced by the vehicle operator than does light from a forward view.
Improvements in glare reduction additionally occurred when prismatic mirrors having two states were replaced with mirror assemblies including dimming elements capable of providing many levels of reflectivity reduction. The most commercially successful type of variable dimming element is the electrochromic dimmer. Electrochromic devices produce various levels of opaqueness in response to an applied voltage. When placed in front of a reflective surface, an electrochromic device provides a variably reflective surface, the degree of reflectivity based on a control voltage. Typical electrochromic mirrors are described in U.S. Pat. No. 4,902,108 to Byker and 5,724,187 to Varaprasad et al. as well as in commonly assigned U.S. Pat. No. 5,928,572 entitled “ELECTROCHROMIC LAYER AND DEVICES COMPRISING SAME” to Tonar et al.; and U.S. Pat. No. 6,030,987 entitled “ELECTROCHROMIC MEDIUM CAPABLE OF PRODUCING A PRE-SELECTED COLOR” to Baumann et al., each of which is hereby incorporated by reference.
A key element in the design of an automatic dimming mirror is the type of light transducer used to implement ambient light and glare detection. A primary characteristic of interest in selecting a light transducer type is the dynamic range. The ratio between the intensity of bright sunlight and moonlight is roughly 1,000,000:1, indicating the wide range that must be sensed by the ambient light sensor. Both the ambient light and the glare light sensors must operate within the ranges of temperature, humidity, shock, and vibration experienced within a vehicle passenger compartment. If a sensor is to be mounted in an outside mirror, even harsher operating conditions can be expected. Sensors and support electronics must also be inexpensive to allow the cost of an automatically dimmed mirror to fall within the range deemed acceptable by an automobile purchaser. Transducers should have good noise immunity or be compatible with noise compensation electronics within the sensor for sensitivity at low light levels. Transducers should further have a spectral response similar to the frequency response of the human eye. As a final desirable characteristic, the sensor must be easily integratable into the types of digital control systems commonly found in automotive applications.
One type of light transducer is the cadmium sulfide (CdS) cell. CdS cells are photosensitive resistors exhibiting increasing conductance with increasing light levels. CdS cells have the advantages of being low in cost, having good sensitivity to low light levels, and having a spectral response somewhat similar to that of the human eye. Disadvantages with CdS cells include a high degree of variance between cells, slow response at low light levels, poor environmental stability, and difficulty being assembled by automated electronic manufacturing equipment. CdS cells for sensing ambient light and glare may be incorporated into a full or partial bridge to increase dynamic range. However, the bridge output represents a fixed relationship between ambient light and glare that is often not appropriate throughout the range of ambient light levels.
Another type of light transducer used in automatic dimming mirrors is the discrete photodiode configured as a light-dependent current source. Photodiodes have less variance between parts, better environmental stability, and are more easily adapted to automated manufacturing than are CdS cells. However, photodiodes tend to be expensive and produce very low currents at low light levels. These low currents require special amplification techniques to achieve a useful signal, increasing the cost of the automatic mirror.
A relatively new type of light sensor incorporates a silicon-based light transducer and conditioning electronics on a single substrate. The light transducer generates charge at a rate proportional to the amount of incident light. This light-induced charge is collected over an integration period. The resulting potential indicates the level of light to which the sensor is exposed over the integration period. Light sensors with integral charge collection have many advantages. By varying the integration time, the sensor dynamic range is greatly extended. Also, the ability to incorporate additional electronics on the same substrate as the transducer increases noise immunity and permits the sensor output to be formatted for use by a digital circuit. Component integration additionally reduces the system cost. Silicon devices are more temperature invariant than CdS cells and can be packaged to provide the necessary protection from humidity, shock, and vibration. One disadvantage of silicon-based light transducers is a frequency response different than that of the human eye. Types of charge accumulating light transducers include photodiodes and photogate transistors. A variety of charge integrating photodiode devices have been described including those in U.S. Pat. Nos. 4,916,307 to Nishibe et al.; U.S. Pat. No. 5,214,274 to Yang; U.S. Pat. No. 5,243,215 to Enomoto et al.; U.S. Pat. No. 5,338,691 to Enomoto et al.; and U.S. Pat. No. 5,789,737 to Street. Photogate transistor devices are described in U.S. Pat. Nos. 5,386,128 to Fossum et al. and U.S. Pat. No. 5,471,515 to Fossum et al. Each of these patents is hereby incorporated by reference.
One difficulty with all types of light sensors is the occurrence of operating anomalies at high temperatures. Some devices become extremely non-linear at high temperatures. Some devices, such as CdS cells, may suffer a permanent change in operating characteristics. Devices may even provide completely false readings such as indicating bright light in low light conditions due to excessive thermal noise. Traditionally, the only way to deal with this problem has been to incorporate a temperature sensor and associated electronics into the automatic dimming mirror.
What is needed is an automatic dimming mirror that derives the benefits provided by semiconductor light sensors with integral charge collection. The mirror should operate over a wide range of lighting conditions and be less susceptible to temperature variations.